This invention relates to the control of cardiac arrhythmias and particularly to terminating fibrillation in the heart. The method uses an improved electrode configuration for use with a single monophasic or biphasic current pulse to effectively lower the energy required to successfully defibrillate the heart.
Ventricular fibrillation is characterized by the random depolarization of individual fibers of the heart which greatly reduces the cardiac output of the heart and leads to death within minutes of onset. Conventional external treatment for fibrillation calls for the application of an electric shock supplied by a pair of paddles across the chest of the patient which simultaneously depolarizes all of the cardiac muscle fibers. This process permits a resynchronization of the ventricular muscle fibers.
Implantable defibrillator systems have been proposed for use in patients susceptible to sudden death syndrome. Traditionally such systems have comprised an implanted pulse generator coupled to a plurality of electrodes located in and around the heart. Such implantation techniques require a thorocotomy to place the electrodes. However, the most fundamental problem associated with implantable defibrillation is the high energy required to successfully defibrillate the heart.
One early attempt to produce an electrode system suitable for use in an automatic implantable defibrillator is illustrated in U.S. Pat. No. 3,942,536. In this system, a single right ventricular endocardial lead is used having one set of electrodes at its distal tip for location in the apex of the right ventricle, and a second set of electrodes spaced from the set of electrodes on the distal tip a sufficient distance to place them outside the heart, in the superior vena cava. Other endocardial ventricular defibrillation lead systems are illustrated in U.S. Pat. No. 3,857,398 issued to Rubin and in U.S. Pat. No. 4,355,646 issued to Kallok.
Experience with these lead systems have shown that the energy required to defibrillate the heart utilizing a single pair of electrodes while significantly less than that required by use of an external defibrillator is still sufficiently large to make construction of a battery powered automatic implantable defibrillator difficult. Additionally, the small electrode areas required by catheter mounted systems have been shown to increase the risk of tissue damage because of the increase current density present at the electrode sites.
In an effort to overcome this problem, electrode systems have been proposed such as that shown in U.S. Pat. No. 4,030,509 to Heilman which shows a collection of large surface area electrodes. One set of electrodes is applied to the apex of the heart, a second set is applied to the atria of the heart. As an alternative, it has been suggested that a superior vena cava electrode on an endocardial lead may also be used in conjunction with the large area electrode applied directly to the apex of the heart. One problem associated with the use of epicardial patches on the heart is that the surgery to attach the electrodes is highly invasive, and therefore undesirable.
Other large surface area electrodes for application to the human heart are disclosed in U.S. Pat. No. 4,291,707 issued to Heilman et al, which discloses electrodes fabricated of metallic mesh, sandwiched between two layers of chemically inert electrically insulative material.
Recently, it has been proposed that rather than delivering electrical energy between electrodes located in the apex of the heart and electrodes located on or in the superior vena cava or atrium of the heart that a return to application of electrical energy transversely across the heart is desirable. For example, in published European Pat. Application Publication No. 0 095 726 by the Purdue Research Foundation, it is proposed that four epicardial mesh electrodes be arranged orthogonally around the heart and that defibrillation be accomplished using two sequential orthogonal defibrillation pulses.
A large portion of the early work on automatic implantable and external defibrillators was performed with stimulators which provided a single monophasic pulse, or a symmetrical biphasic pulse.
In the present invention, a combination of electrode placement and size have been optimized to produce a system capable of defibrillation at lower energies than have been heretofore possible. It appears that the electrode placement achieves sufficient spatial isolation to minimize the risk of myocardial damage. The electrode system also has the major advantage of not requiring a thoracotomy for electrode placement.
The electrode system is used in conjunction with a novel waveform providing a single asymmetric biphasic defibrillation pulse.